What is the recommended course of action for a pregnant woman at 36/37 weeks in labor with a prior positive Group B Streptococcus (GBS) screen at 34 weeks and no treatment?

Immediate IV Penicillin G Prophylaxis Required

This GBS-positive woman in labor at 36/37 weeks must receive intravenous penicillin G (or ampicillin) immediately and continued every 4 hours until delivery—this is the only evidence-based intervention that reduces neonatal morbidity and mortality from early-onset GBS disease. 1

Why IV Antibiotics Are Essential

The CDC explicitly states that women with positive GBS vaginal-rectal screening cultures in late gestation during the current pregnancy require intrapartum antibiotic prophylaxis when labor begins 2, 1. The screening culture remains valid for 5 weeks from collection 2, 1, so her positive result at 34 weeks is still applicable at 36/37 weeks.

Critical point: The CDC guidelines make no distinction between treated and untreated GBS-positive women—both require intrapartum prophylaxis. 1 The fact that she didn't receive "treatment" at 34 weeks is irrelevant because antepartum oral antibiotic treatment of GBS colonization is ineffective in eliminating carriage, does not prevent neonatal disease, and may promote antibiotic resistance 2, 1, 3.

Correct Antibiotic Regimen

First-Line Treatment

• Penicillin G: 5 million units IV initially, then 2.5-3 million units IV every 4 hours until delivery 1, 3
• Alternative: Ampicillin 2 g IV initially, then 1 g IV every 4 hours until delivery 2, 1

For Penicillin Allergy

• Low-risk allergy: Cefazolin 2 g IV initially, then 1 g IV every 8 hours 1
• High-risk allergy: Clindamycin 900 mg IV every 8 hours or vancomycin 1 g IV every 12 hours 1

Why the Other Options Are Wrong

Option A (Immediate C-section): Incorrect

The CDC specifically states that intrapartum prophylaxis is not routinely recommended only for cesarean deliveries performed before labor onset with intact membranes 2, 1. Since this patient is already in labor, cesarean delivery offers no advantage over vaginal delivery with appropriate antibiotic prophylaxis and would expose her to unnecessary surgical risks.

Option B (Single dose oral amoxicillin): Incorrect

This violates multiple evidence-based principles:

• Oral antibiotics are explicitly contraindicated for GBS prophylaxis 2
• Effective prophylaxis requires intravenous administration for adequate tissue and fetal blood levels 1
• Effective prophylaxis requires multiple doses continued until delivery, not a single dose 1

Effectiveness Evidence

Beta-lactam prophylaxis given 4 or more hours before delivery is highly effective for prevention of early-onset GBS disease, with 91% effectiveness among term neonates and 86% effectiveness among preterm neonates 4. Even shorter durations provide benefit: prophylaxis of 2 to fewer than 4 hours showed 47% effectiveness, and fewer than 2 hours showed 38% effectiveness 4.

Importantly, penicillin G levels increase linearly until 1 hour after administration and all subgroups analyzed achieved levels 10-179 fold above the minimal inhibitory concentration for GBS, suggesting benefit even in precipitous labors 5.

Common Pitfalls to Avoid

• Never rely on antepartum oral antibiotic treatment—it does not eliminate GBS colonization or prevent neonatal disease 2, 1, 3
• Do not delay prophylaxis—start IV antibiotics as soon as labor is confirmed 2, 1
• Avoid clindamycin unless truly indicated for high-risk penicillin allergy—its effectiveness is significantly lower at only 22% compared to 91% for beta-lactams 4
• Continue antibiotics every 4 hours until delivery—penicillin G does not accumulate and returns to baseline after each 4-hour interval 5